Disk drives store information on magnetic disks. Typically, the information is stored in concentric tracks on the disk and the tracks are divided into servo sectors that store servo information and data fields that store user data. A transducer head reads from and writes to the disk. The transducer head is mounted on an actuator arm assembly that moves the transducer head radially over the disk. Accordingly, the actuator arm assembly allows the transducer head to access different tracks on the disk. The disk is rotated by a spindle motor at high speed, allowing the transducer head to access different data fields within each track on the disk.
FIG. 1 illustrates a disk drive 100 that includes a base 104 and a magnetic disk (or disks) 108 (only one of which is shown). The disk 108 is connected to the base 104 by a spindle motor (not shown) mounted within or beneath a hub 112 such that the disk 108 rotates relative to the base 104. An actuator arm assembly 116 is connected to the base 104 by a bearing 120 and suspends a transducer head 124 at a first end. The transducer head 124 reads data from and writes data to the disk 108. A voice coil motor 128 pivots the actuator arm assembly 116 about the bearing 120 to radially position the transducer head 124 relative to the disk 108. By changing the radial position of the transducer head 124 relative to the disk 108, the transducer head 124 accesses different tracks 132 on the disk 108. The voice coil motor 128 is operated by a controller 136 that is operatively connected to a host computer (not shown). A channel 140 processes data read from the disk 108 by the transducer head 124.
FIG. 2 illustrates the disk 108 in more detail. The tracks 132 are divided into data fields 204a–204h and servo sectors 208a–208h. The data fields 204a–204h store user data and the servo sectors 208a–208h store servo information to provide the transducer head 124 with its radial position over the disk 108.
Although the disk 108 has a relatively small number of tracks 132, data fields 204 and servo sectors 208, a typical disk contains a very large number of tracks, data fields and servo sectors. For example, disks having over 30,000 tracks per inch and 120 servo sectors per track are presently available. In addition, alternate configurations of the disk 108 are possible. For example, one surface of the disk 108 can be dedicated to servo information while the other surface of the disk 108 (and any remaining disks 108 in the disk drive 100) can exclusively store user data.
Data is stored on the disk 108 using data patterns with magnetic transitions between opposite magnetic polarities. For example, the magnetic polarity in a first direction encodes a digital 1, and the magnetic polarity in a second direction encodes a digital 0. A bit cell is the shortest length of the track 132 to which a particular magnetic polarity is written. Accordingly, a magnetic transition from one bit cell to the next bit cell indicates a change from one digital character to another.
The disk 108 is formed by depositing a magnetic film on a rigid substrate. The thickness of the magnetic film must be closely controlled. Where the magnetic film is too thin, the magnetic flux density produced by a magnetic transition will be too weak. The disk 108 may also contain other defects, such as scratches or pits, that degrade the magnetic flux density produced by a magnetic transition. These defects can occur during the manufacture of the disk 108 or during the assembly of the disk drive 100.
The disk drive 100 is subject to numerous qualification tests to ensure reliable storage and retrieval of user data once delivered to an end user. Flaw scan is one such qualification test. Flaw scan identifies areas of the disk 108 that may not reliably store user data. Flaw scan writes a data pattern to the data fields 204 (and any other writable areas of the disk 108) and then reads the data pattern from the data fields 204 (and any other writable areas of the disk 108) following assembly of the disk drive 100. The magnetic polarity in the data pattern can alternate every bit cell to produce a 1T data pattern, or every ith bit cell to produce an iT data pattern where i is an integer. For instance, the magnetic polarity can alternate every two bit cells to produce a 2T data pattern (110011001100 . . . ), every three bit cells to produce a 3T data pattern (111000111000 . . . ) and so on.
The transducer head 124 generates a read signal in response to reading the data pattern from the disk 108, and the read signal includes pulses caused by the magnetic transitions in the data pattern. The isolated pulse width (PW50) is the distance between the points of intersection between an isolated pulse and a line indicating 50% of the maximum amplitude of the isolated pulse. Intersymbol interference is the alteration of an isolated pulse due to linear superposition of other pulses in close proximity.
Data patterns with long periods (iT) that occupy a length of the track 132 that is greater than the PW50 of a read signal derived from the disk 108 cause the transducer head 124 to generate a read signal with greater amplitude due to decreased intersymbol interference. Alternatively, data patterns with short periods that occupy a length of the track 132 that is less than the PW50 increase the likelihood of detecting a flaw or the inability of a particular length of the track 132 to produce the prescribed magnetic flux density.
The channel 140 includes a partial response maximum likelihood (PRML) detector (not shown) that accurately detects the data patterns even when the user data is written on the disk 108 at high bit density and the read signal exhibits intersymbol interference. The PRML detector samples the read signal at regular time intervals and determines a code word that symbolizes a set of pulses using a statistical maximum likelihood or Viterbi process. For instance, the PRML detector detects a data pattern when the PW50 contains 2.5 bits of information. Accordingly, the PRML detector allows user data to be recorded at higher density than a peak detector since the peak detector is incapable of reliably decoding pulses with intersymbol interference.
The channel 140 often uses a 2T preamble to synchronize sample times (phase) and determine signal amplitudes to adjust the gain. When the phase and gain are properly adjusted, a 2T sampled waveform in the channel 140 produces a distinctive pattern. Furthermore, flaw scan often uses a 2T data pattern because of the high magnetic transition rate, low intersymbol interference, availability in the channel 140 and unique sampled pattern it produces in the channel 140.
FIG. 3 is a flow chart of a conventional flaw scan. The transducer head 124 writes a data pattern to the data fields 204 (step 300) and then reads the data pattern from the data fields 204 to obtain n−1 samples (step 304) and then a next sample (the nth sample) (step 308). The channel 140 serially determines whether each of the previous n samples have an amplitude that is less than a threshold value (step 312). If at least one of the previous n samples has an amplitude that is greater than the threshold value, then the channel 140 returns to step 308 to take a next sample. Otherwise, the channel 140 reports a flaw to the controller 136 (step 316) and returns to step 308 to take a next sample.
Conventional flaw scan is susceptible to erroneously qualifying a series of bit cells where noise or some other disturbance causes one or more samples to exceed the threshold value. As a result, areas of the disk 108 that cannot reliably store user data may nonetheless be qualified. Although the disk drive 100 uses error correction code (ECC) to tolerate some errors, the storage reliability could still be compromised. Similarly, conventional flaw scan is susceptible to erroneously disqualifying a length of the track 132 that does not contain errors in the presence of a sustained noise event that causes a series of samples to fall below the threshold value. This unnecessarily reduces the storage capacity of the disk drive 100.
Conventional flaw scan typically makes two or more passes over each surface of every disk 108 in the disk drive 100 to reduce soft errors caused by random noise and thus increase the likelihood that flaws will be detected and decrease the likelihood that false errors will be reported. However, multiple flaw scans increase manufacturing time and decrease manufacturing throughput.
There is, therefore, a need for a flaw scan that detects flaws and avoids false errors with high confidence with fewer passes and is inexpensive to implement.